Electrical capacitor manufacturers frequently utilize aluminum foil in the production of their capacitors to reduce both the weight and overall size of the capacitors. The aluminum foil provides a large surface area in a relatively small amount of space within the capacitor container. This surface area of the aluminum foil is usually enhanced through the use of chemical or electrochemical etching of the foil to develop what are known as submicron-diameter tunnel networks. The increased surface area provided by such etching, in combination with the electrical insulation provided by a subsequently formed anodic film on the surface of the foil, is responsible for high capacitance levels attained by the foil per unit area of foil.
Generally speaking, only the relatively expensive high purity aluminum is utilized as the foil material in capacitors. The etching response of such foil depends upon the conditions set forth by the capacitor manufacturer and is determined by both the foil composition and the processing utilized to make the foil. For low voltage applications (below 100 volts) where a leakage current from the capacitor is not as crucial as high voltage applications, the lower purity and less expensive aluminum grades such as AA 1188 or 1193 are generally employed. The cold rolled -H19 temper is preferred over the -0 temper condition for foil utilized in this particular voltage range. This preferance is due to the finer scale tunnel networks which develop on the surface of the foil with cold worked tempers. A full advantage of the etched tunnel networks can be realized in the low voltage applications since the anodic films employed in the capacitor foil manufacture generally do not completely seal the tunnel networks.
Capacitor manufacturing is a very highly cost competitive industry and metal suppliers who can provide aluminum foil which is capable of attaining high capacitance for the least amount of metal cost utilized in the foil have a greater sales advantage over competitors. One method for providing such increased capacitance in fairly low purity aluminum foils is the addition of certain elements which act to enhance the capacitance of the alloys to which they are added. For example, U.S. Pat. No. 3,498,765 describes a 78% improvement in capacitance levels over high purity aluminum utilizing a 70 ppm (parts per million) cadmium addition to the aluminum. This patent further discusses a 10% improvement in capacitance over high purity aluminum through the addition of 60 ppm indium to the base aluminum. U.S. Pat. No. 3,578,570 describes enhanced etch pore density and, therefore, increased capacitance in annealed aluminum foil which contains one or more of the elements antimony, barium or zinc in quantities of 5 to 200 ppm and at most 0.5 ppm of lead and bismuth plus at most 2 ppm of calcium and chromium. However, intentional alloying additions tend to increase the cost of the final product.
It is therefore an object of the present invention to provide an improved aluminum foil which exhibits good capacitance levels in cold worked tempers.
It is a further object of the present invention to provide a foil as above which attains this good capacitance at a low cost and without the necessity for the utilization of specific elemental additions to the aluminum.
It is a further object of the present invention to provide a foil as aforesaid which is based on a composite aluminum base alloy system.
Further objects and advantages of the present invention will become more apparent from a consideration of the following specification.